Why do i bite down on teeth while sleeping?

Bruxism: You may be referring to bruxism during sleep, which is excessive grinding of the teeth ; clenching of the jaw leading to erosion of the dentition. It can be caused by many factors including but not limited to stress, emotional disturbances, certain sleep disorders such as obstructive sleep apnea, anxiety, ; even certain medications. It may be a good idea to consult a sleep specialist ; a dentist.